This invention relates to power supplies and more particularly to a solid state, high efficient supply which converts D.C. energy to high frequency A.C. energy for the purpose of supplying gas discharge tubes with high voltage at relative low currents in a range of 15-55 milliamperes (ma) in a range of 15-115 watts. The high voltage may vary from one kilovolt to 10 kilovolts depending on the glass diameter, length, bends, type of gas, etc.
Upon ionization of a gas discharge tube by means of high voltage resulting in current flow, the atoms of neon are stimulated to emit an orange-red light. Other gases which glow when electrically energized are mercury vapor (blue-green), argon (pale blue), and a mixture of the two (deep blue). Pigmented fluorescent coatings are used with mercury vapor gas to produce many visible hues of light quite efficiently.
One type of prior art power supply is simply 60 Hz transformers where 120 volts A.C. is applied to the primary of the transformer and the secondary winding output voltage is connected to the tube load. By utilizing a large ratio of primary secondary turns such as 50-100, high voltages are induced up to 10 kilovolts. Such systems are heavy, for example 10-12 pounds, dangerous, and may be as inefficient as 85% resulting in high internal temperatures and low reliability. Several sizes of transformers are available to prevent an underdrive or overdrive of the tube load.
More recent solid state power supplies are lighter, more efficient, and operate silently compared with the 120 Hz audible noise from 60 Hz power supplies. However, specific problems are evident with such power supplies, such as: (a) the series resonant type of oscillators employed result in a "beading" of the energized neon gas which is displeasing to the eye; (b) the lack of secondary short circuit protection so the system can fail when the secondary is shorted; (c) the lack of open circuit protection resulting in high voltages up to 16 kilovolts which is dangerous and may result in an arc and a fire; (d) the lack of protection from an open secondary lead or a broken tube which can cause a fire; (e) inadequate protection of persons who may come in contact with the high voltage by touching one of the leads; (f) the absence of a method to set and regulate the amplitude of current to a gas discharge tube often results in failing the tube load; and (g) the absence of circuit capability to connect a millampmeter for the purpose of adjusting the load current to a safe value.
It has been found that tubes filled with mercury vapor gas tend to degrade when excess current is allowed to flow in the tubes due to excessive voltage. For example, such degradation has been observed in window neon signs with currents which exceed the nominal current by only 20%. The general symptom resulting from current overdrive is a dimming or darkening of specific sections of the tube caused by condensation of the mercury vapor which results in reducing the secondary emission of light from the flourescent coating.
Gas discharge tubes have a negative coefficient of resistance with current. That is, the tube's resistance decreases as the current through it increases which suggests that a runaway condition exists if the current is not regulated.
The glass used for window neon signage range from 9-12 mm. High voltage, gas discharge tubes used for lighting are generally 15 or 18 mm's, are filled with mercury gas, and emit white light. The area of the glass inside diameter determines the amount of high voltage and resultant current which will be tolerated by mercury vapor sections of signs or lighting systems. In commercial practice, the outside diameter of the glass is used as reference rather than the inside diameter. The following table illustrates the nominal and damaging currents for lighting devices of various sizes.
______________________________________ Use Ranqe mm Optimum ma Damaging ma ______________________________________ Sign 8-9 20 24 Sign 9-10 22 26 Sign 10-11 24 28 Sign 11-12 26 31 Lighting 15 34 41 Lighting 18 41 49 ______________________________________
Neon gas tubing is not easily damaged by excessive voltage and resultant current, however neon and mercury vapor sections generally are arranged in series in signage resulting in the need for regulation of the current because of the mercury vapor sections. Also, when more than one section of tubing is used to configure the sign, such as four sections of different colors, the smallest diameter mercury vapor section determines the safe current limit. Often tubes are bent sharply during the manufacturing process resulting in reducing the area of the tube at these points by the equivalent of 1-2 mm's.